Particle simulation for 10.6 micron picosecond laser-induced breakdown in gases

摘要

A theoretical model was developed to explain experimental observations in free electron laser experiments: argon plasmas were initiated and sustained by a free electron laser operating at a wavelength of 10.6 $mu m$ with a sequence of 10 ps pulses, and attempts to obtain optical breakdown with the same laser in nitrogen and hydrogen proved unsuccessful. The particle simulation method was used in this model to calculate the electron density increase and the electron energy distribution during the gas breakdown process. A Monte Carlo collision method was incorporated into the particle simulation method to calculate collisions between particles. The theoretical model also included three-body recombination and gasdynamic expansion effects. The calculated results showed that the argon plasma breakdown in the free electron laser experiments was the result of a cumulative growth of the electron density during 27 pulses. Hydrogen and nitrogen did not reach breakdown at the free electron laser experiment conditions because the electron energy decayed more rapidly in these gases during the interval between pulses. The reduction of electron energy permits a much faster electron recombination rate which depletes the electron density during the interval between pulses. The particle simulation predicts that hydrogen and nitrogen can reach breakdown at a power threshold which is higher than that which was used in the free electron laser experiments.